1. Field of the Invention
The invention relates to a diagnostic system and a diagnostic method for a vehicle that is driven by an internal combustion engine and, more particularly, to a system and a method that diagnoses the fuel efficiency of a vehicle.
2. Description of the Related Art
Various methods of calculating the fuel efficiency (that is, the relationship between a fuel consumption and a mileage) of a vehicle, particularly, a vehicle driven by an internal combustion engine, are suggested in Japanese Patent Application Publication No. 2006-183506 (JP-A-2006-183506), Japanese Patent Application Publication No. 2004-60548 (JP-A-2004-60548), Japanese Patent Application Publication No. 2006-118480 (JP-A-2006-118480), Japanese Patent Application Publication No. 1-290967 (JP-A-1-290967), and the like. For example, JP-A-2006-183506 describes a method of calculating the fuel efficiency on the basis of a fuel injection amount and a torque of the internal combustion engine. With the above method, the output of the internal combustion engine is computed on the basis of a torque and rotational speed of the internal combustion engine in a predetermined period of time, a total fuel supply amount in the predetermined period of time is computed, and then the fuel efficiency is computed on the basis of the relationship between the output of the internal combustion engine and the total fuel supply amount.
Incidentally, factors that influence the fuel efficiency of the vehicle include the service condition of the vehicle, the operating condition of a driver and the state of the vehicle itself. The method of calculating the fuel efficiency described in JP-A-2006-183506 is a method of calculating a comprehensive fuel efficiency that includes all of these factors.
However, when remedial measures for fuel efficiency are attempted to be taken on the basis of the results of calculation, the content of the remedial measures differs depending on whether the service condition of the vehicle or the operating condition of the driver influences the fuel efficiency or whether the state of the vehicle itself (for example, degradation of the internal combustion engine due to changes over time) influences the fuel efficiency. For example, when the fuel efficiency deteriorates because of the service condition of the vehicle or the operating condition of the driver, the fuel efficiency may be improved by reconsidering the service condition or changing the way of driving. In contrast, when the fuel efficiency deteriorates because of the state of the vehicle itself, it is necessary to inspect and maintain the vehicle for improvement in fuel efficiency, and there is a possibility that the fuel efficiency may not be improved so much depending on the progress of degradation of the vehicle. Thus, it is impossible to determine whether deterioration of the fuel efficiency depends on the service condition of the vehicle or the operating condition of the driver or depends on the state of the vehicle itself.
Note that, for example, JP-A-2004-60548 describes a method of calculating the fuel efficiency using a specific fuel consumption map that defines the relationship among a rotational speed of the internal combustion engine, a torque and a specific fuel consumption (g/(kW·h)). With the above method, a correction factor of a current specific fuel consumption with respect to an initial specific fuel consumption of the internal combustion engine is calculated on the basis of a mileage, and the correction factor of the specific fuel consumption is multiplied by values stored in the specific fuel consumption map to thereby correct the specific fuel consumption map. Then, a specific fuel consumption of the internal combustion engine is computed on the basis of the rotational speed and torque of the internal combustion engine by referring to the corrected specific fuel consumption map, and a fuel consumption of the internal combustion engine is computed on the basis of the computed specific fuel consumption.